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The Spectrum of Markarian 421 Above 100 GeV with STACEE

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1 The Spectrum of Markarian 421 Above 100 GeV with STACEE
Jennifer Carson UCLA / Stanford Linear Accelerator Center February 2006 Space-based Introduce myself. Gamma-ray spectrum covers several decades… List (briefly) different types of experiments… Focus on STACEE 1 MeV 10 MeV 100 MeV 1 GeV 10 GeV 100 GeV 1 TeV 10 TeV Ground-based

2 Outline I. Science goal for -ray detections from active galaxies
• Understanding particle acceleration II. Brief overview of VHE gamma-ray astronomy III. Ground-based detection techniques • Atmospheric Cherenkov technique • Imaging vs. wavefront sampling IV. STACEE V. Markarian 421 • First STACEE spectrum • Detection of high-energy peak? VI. Prospects for the future

3 Radio-loud AGN viewed at small angles to the jet axis
Blazars Radio-loud AGN viewed at small angles to the jet axis • bright core •  3% optical polarization • strong multi-wavelength variability

4 Radio-loud AGN viewed at small angles to the jet axis
Blazars Radio-loud AGN viewed at small angles to the jet axis • bright core •  3% optical polarization • strong multi-wavelength variability • Double-peaked SED: synchrotron emission + ? 10-3 eV (radio) 1 eV (IR) 1 keV (X-ray) 1 MeV 1 GeV ? Maraschi et al. 1994 synchrotron What physical processes produce the high-energy emission?

5 Blazar High-Energy Emission Models
Is the beam particle an e- or a proton? The “standard” picture of an AGN: a central SMBH accretes material from a close-in disk and funnels material into relativistic jets. Two competing models for gamma-ray emission from AGN: Inverse compton (SSC or ECS) Proton-induced cascades The two models make very different predictions for the spectrum of the high-energy emission – much more emission above ~100 GeV in the hadronic models.

6 Blazar High-Energy Emission Models
Is the beam particle an e- or a proton? Leptonic models: • Inverse-Compton scattering off accelerated electrons • Synchrotron self-Compton and/or • External radiation Compton The “standard” picture of an AGN: a central SMBH accretes material from a close-in disk and funnels material into relativistic jets. Two competing models for gamma-ray emission from AGN: Inverse compton (SSC or ECS) Proton-induced cascades The two models make very different predictions for the spectrum of the high-energy emission – much more emission above ~100 GeV in the hadronic models.

7 Blazar High-Energy Emission Models
Is the beam particle an e- or a proton? Leptonic models: • Inverse-Compton scattering off accelerated electrons • Synchrotron self-Compton and/or • External radiation Compton Hadronic models: • Accelerated protons • Gamma rays from pion decay or • Synchroton gamma-ray photons The “standard” picture of an AGN: a central SMBH accretes material from a close-in disk and funnels material into relativistic jets. Two competing models for gamma-ray emission from AGN: Inverse compton (SSC or ECS) Proton-induced cascades The two models make very different predictions for the spectrum of the high-energy emission – much more emission above ~100 GeV in the hadronic models.

8 Blazar High-Energy Emission Models
Is the beam particle an e- or a proton? Leptonic models: • Inverse-Compton scattering off accelerated electrons • Synchrotron self-Compton and/or • External radiation Compton Hadronic models: • Accelerated protons • Gamma rays from pion decay or • Synchroton gamma-ray photons The “standard” picture of an AGN: a central SMBH accretes material from a close-in disk and funnels material into relativistic jets. Two competing models for gamma-ray emission from AGN: Inverse compton (SSC or ECS) Proton-induced cascades The two models make very different predictions for the spectrum of the high-energy emission – much more emission above ~100 GeV in the hadronic models. Gamma-ray observations around 100 GeV can distinguish between models

9 Exploring the Gamma-ray Spectrum
Atm. Cherenkov Wavefront Sampling Atm. Cherenkov Single Dish Imaging Past Compton Gamma Ray Observatory Air shower arrays 100 MeV 1 GeV 10 GeV 1 TeV TeV Energy 1 MeV Atm. Cherenkov Wavefront Sampling Present/Future Atm. Cherenkov Imaging Arrays GLAST

10 Atmospheric Cherenkov Technique
g-ray e+ e- … q ~ 1.5o

11 Single-dish Imaging Telescopes
g-ray q ~ 1.5o Whipple 10-meter

12 Wavefront Sampling Technique
g-ray q ~ 1.5o

13 Wavefront Sampling Technique
g-ray q ~ 1.5o

14 Exploring the Gamma-ray Spectrum
Atm. Cherenkov Wavefront Sampling Atm. Cherenkov Single Dish Imaging Past Compton Gamma Ray Observatory Air shower arrays 100 MeV 1 GeV 10 GeV 1 TeV TeV Energy 1 MeV Atm. Cherenkov Wavefront Sampling Present/Future Atm. Cherenkov Imaging Arrays GLAST Whipple 10-meter

15 Exploring the Gamma-ray Spectrum
Atm. Cherenkov Wavefront Sampling Atm. Cherenkov Single Dish Imaging Air shower arrays STACEE 100 MeV 1 GeV 10 GeV 1 TeV TeV Energy 1 MeV Atm. Cherenkov Wavefront Sampling Atm. Cherenkov Imaging Arrays GLAST Whipple 10-meter CELESTE

16 Exploring the Gamma-ray Spectrum
Atm. Cherenkov Wavefront Sampling Atm. Cherenkov Single Dish Imaging Air shower arrays STACEE 100 MeV 1 GeV 10 GeV 1 TeV TeV Energy 1 MeV Atm. Cherenkov Wavefront Sampling Present/Future Atm. Cherenkov Imaging Arrays GLAST

17 Exploring the Gamma-ray Spectrum
STACEE HESS 100 MeV 1 GeV 10 GeV 1 TeV TeV Energy 1 MeV Atm. Cherenkov Wavefront Sampling Present/Future Atm. Cherenkov Imaging Arrays GLAST VERITAS

18 The High-Energy Gamma Ray Sky
1995  3 sources Mrk421 Mrk501 Crab (2) Pulsar Nebula (1) AGN (0) SNR Other, UNID (0) R.A.Ong Aug 2005

19 The High-Energy Gamma Ray Sky
2004  12 sources Mrk421 H1426 M87 Mrk501 1ES1959 RXJ 1713 Cas A GC TeV 2032 Crab 1ES 2344 PKS 2155 (7) Pulsar Nebula (1) AGN (2) (1,1) SNR Other, UNID R.A.Ong Aug 2005

20 The High-Energy Gamma Ray Sky
2005  31 sources! add. sources in galactic plane. 1ES 1218 Mrk421 H1426 M87 Mrk501 PSR B1259 1ES 1101 1ES1959 SNR G0.9 RXJ 1713 RXJ 0852 Cas A LS 5039 GC Vela X TeV 2032 Crab 1ES 2344 HessJ1303 Cygnus Diffuse PKS 2155 MSH 15-52 H2356 PKS 2005 Pulsar Nebula (4+2) (11) AGN (3+4) (3,3+1) SNR Other, UNID R.A.Ong Aug 2005

21 Wavefront Sampling Detectors
STACEE & CELESTE e+ e- … Properties of gamma-ray showers: Light distribution is fairly uniform Energy is evenly divided between bremsstrahlung photons and kinetic energy of e-s Cherenkov light intensity on ground  energy of gamma ray Cherenkov pulse arrival times at heliostats  direction of source

22 STACEE Solar Tower Atmospheric Cherenkov Effect Experiment 5 secondary
64 heliostats 5 secondary mirrors & 64 PMTs National Solar Thermal Test Facility Albuquerque, NM Unique method: only one instrument like this has published results (CELESTE) CELESTE was more sensitive than we are, but had terrible weather Cosmic rays are a significant background, at least 1000 times as bright as the Crab

23 STACEE Solar Tower Atmospheric Cherenkov Effect Experiment 5 secondary
64 heliostats 5 secondary mirrors & 64 PMTs National Solar Thermal Test Facility Albuquerque, NM • PMT 4 PEs: ~10 MHz • Two-level trigger system (24 ns window): - cluster: ~10 kHz - array: ~7 Hz • 1-GHz FADCs digitize each Cherenkov pulse Unique method: only one instrument like this has published results (CELESTE) CELESTE was more sensitive than we are, but had terrible weather Cosmic rays are a significant background, at least 1000 times as bright as the Crab

24 STACEE Advantages / Disadvantages
• 2-level trigger system  good hardware rejection of hadrons • GHz FADCs  pulse shape information • Large mirror area (6437m2)  low energy threshold Ethresh = 165 GeV Hardware CR rejection: HESS is running at 300 Hz, Whipple would be running at that if they tried to push to our energies, we run at 7 Hz because of hardware CR rejection Energy threshold is currently limited by width of coincidence window and threshold -> NSB contamination Compare sensititivity to 5 sigma in an hour with Whipple, 5 sigma in 30 seconds (!) with HESS

25 STACEE Advantages / Disadvantages
• 2-level trigger system  good hardware rejection of hadrons • GHz FADCs  pulse shape information • Large mirror area (6437m2)  low energy threshold Ethresh = 165 GeV But… • Limited off-line cosmic ray rejection  limited sensitivity: 1.4/hour on the Crab Nebula • Compare to Whipple sensitivity: 3/hour above 300 GeV Hardware CR rejection: HESS is running at 300 Hz, Whipple would be running at that if they tried to push to our energies, we run at 7 Hz because of hardware CR rejection Energy threshold is currently limited by width of coincidence window and threshold -> NSB contamination Compare sensititivity to 5 sigma in an hour with Whipple, 5 sigma in 30 seconds (!) with HESS

26 STACEE Data Observing Strategy: Equal-time background observations for
every source observation (1-hour “pairs”) Analysis: Cuts for data quality Correction for unequal NSB levels Cosmic ray background rejection Significance/flux determination

27 Energy Reconstruction Idea
Utilize two properties of gamma-ray showers: Linear correlation: Cherenkov intensity and gamma-ray energy Uniform intensity over shower area 200 GeV gamma ray 500 GeV proton

28 Energy Reconstruction Method
New method to find energies of gamma rays from STACEE data: 1. Reconstruct Cherenkov light distribution on the ground from PMT charges 2. Reconstruct energy from spatial distribution of light Results: fractional errors < 10% energy resolution ~25-35%

29 Markarian 421 • Nearby: z = 0.03 • First TeV extragalactic source detected (Punch et al. 1992) • Well-studied at all wavelengths except GeV • Inverse-Compton scattering is favored • High-energy peak expected around 100 GeV • Only one previous spectral measurement at ~100 GeV (Piron et al. 2003) Blazejowski et al. 2005 STACEE Krawczynski et al. 2001 STACEE ? log (E2 dN/dE / erg cm-2 s-1) log (Energy/eV)

30 STACEE Detection of Mkn 421
• Observed by STACEE January – May 2004 • 9.1 hours on-source + equal time in background observations • Non – Noff = 2843 gamma-ray events • 5.8 detection • 5.52  0.95 gamma rays per minute • Energy threshold ~198 GeV for  = 1.8 Pairwise distribution of  Eth = 198 GeV  = 1.8

31 Spectral Analysis of Mkn 421
• Six energy bins between 130 GeV and 2 TeV • Find gamma-ray excess in each bin • Convert to differential flux with effective area curve Gamma-ray rate (photons s-1 GeV-1) Effective area (m2) Rate (photons s-1 GeV-1) Aeff(E) (m2) Energy (GeV) Energy (GeV)

32 Spectral Analysis of Mkn 421
First STACEE spectrum  = 1.8  0.3stat

33 Spectral Analysis of Mkn 421
First STACEE spectrum  = 1.8  0.3stat Flat SED

34 2004 Multiwavelength Campaign
January February March April TeV X-ray PCA data courtesy of W. Cui, Blazejowski et al. 2005

35 2004 Multiwavelength Campaign
January February March April TeV X-ray STACEE observations PCA data courtesy of W. Cui, Blazejowski et al. 2005 • STACEE coverage: 40% of MW nights • ~90% of STACEE data taken during MW nights • STACEE combines low and high flux states

36 Multiwavelength Results
Blazejowski et al. 2005

37 Multiwavelength Results
STACEE region Blazejowski et al. 2005

38 STACEE + Whipple Results

39 STACEE + Whipple Results

40 Science Interpretation
• STACEE’s first energy bin is ~90 GeV below Whipple’s. • STACEE result:  is consistent with a flat or rising SED.  suggests that the high-energy peak is above ~200 GeV. slghtly is inconsistent with most past IC modeling. • Combined STACEE/Whipple data suggest that the peak is around GeV. • SED peak reflects peak of electron energy distribution.

41 Prospects for the Future
• STACEE will operate for another year Air Cherenkov Single Dish Imaging 100 MeV 1 GeV 10 GeV 1 TeV TeV Energy 1 MeV Compton Gamma Ray Observatory Wavefront Sampling Gamma-ray spectrum

42 Prospects for the Future
• STACEE will operate for another year • Imaging arrays coming online, Ethreshold  150 GeV - HESS: 5 Crab detection in 30 seconds! - VERITAS: 2 (of 4) dishes completed Gamma-ray spectrum Air Cherenkov Wavefront Sampling Air Cherenkov Imaging Arrays Compton Gamma Ray Observatory 1 MeV 10 MeV 100 MeV 1 GeV 10 GeV 100 GeV 1 TeV 10 TeV Energy

43 VHE Experimental World
MILAGRO TIBET STACEE MAGIC TIBET ARGO-YBJ MILAGRO STACEE TACTIC PACT VERITAS GRAPES TACTIC HESS CANGAROO

44 Prospects for the Future
• STACEE will operate for another year • Imaging arrays coming online, Ethreshold  150 GeV - HESS: 5 Crab detection in 30 seconds! - VERITAS: 2 (of 4) dishes completed • GLAST launch in 2007! Gamma-ray spectrum Air Cherenkov Wavefront Sampling Air Cherenkov Imaging Arrays Compton Gamma Ray Observatory GLAST 1 MeV 10 MeV 100 MeV 1 GeV 10 GeV 100 GeV 1 TeV 10 TeV Energy

45 GLAST Large Area Telescope (LAT)
Burst Monitor (GBM) • Two instruments:  LAT: 20 MeV – >300 GeV   GBM: 10 keV – 25 MeV • LAT energy resolution ~ 10% • LAT source localization < 0.5’

46 EGRET Sources

47 GLAST Potential

48 Conclusions • Gamma-ray observations of AGN are key to understanding
particle acceleration in the inner jets • Many new VHE gamma-ray detectors & detections • STACEE - “1st-generation” instrument sensitive to ~100 GeV gamma rays - Energy reconstruction is successful - 5.8 detection of Markarian 421 - Preliminary spectrum between 130 GeV and 2 TeV - Second spectrum of Mkn 421 at GeV - High-energy peak is above ~200 GeV • Bright future for gamma-ray astronomy

49 Cosmic Ray Background Rejection
What we have: • Hardware hadron rejection (~103) • Off-source observations for subtraction • 2 from shower core reconstruction (‘templates’) • Timing information & 2 from wavefront fit • RMS on average # photons at a heliostat • Limited directional information - reconstruction precision ~0.2°, FOV ~0.6° Some initial success with the Crab nebula… • 5.4 hours on-source after data quality cuts • 3.0 before hadron rejection • Cut on core fit 2 + wavefront fit 2 + direction: 5.7!

50 Field Brightness Correction
Extra light in FOV from stars will increase trigger rate due to promotions of sub-threshold cosmic rays

51 Field Brightness Correction
Extra light in FOV from stars will increase trigger rate due to promotions of sub-threshold cosmic rays After correction Correct using information from FADCs to equalize light levels

52 STACEE Atmospheric Monitor
• Goal: measure atmospheric transmission and detect clouds. • Meade 8'' S-C scope, Losmandy equatorial mount with PC-controlled “goto” pointing/tracking, SBIG CCD camera for pointing and photometry. • Two IR radiometers (cloud detectors). • Full Weather station.

53 Methods for Finding the Shower Core
1. Finding the centroid • If we sampled the entire shower, we could find its centroid. • The early part of the shower is contained within the array. • Use the first few nanoseconds of the shower to find the centroid. x-position of core y-position of core 2. ‘Template’ method • # PEs vs. shower core position • One template per PMT and energy • Fit core and energy with maximum likelihood estimator • Pros: - precise - 2 for hadron rejection • Con: ‘black box’ Sample template

54 Calibrating the Detector
STACEE Atmospheric Monitor Air shower simulations & Atmospheric monitoring Properties of gamma-ray showers: Light distribution is fairly uniform Energy is evenly divided between bremsstrahlung photons and kinetic energy of e-s

55 Calibrating the Detector
STACEE Atmospheric Monitor Heliostat Secondary PMT DTIRC Trasmission measurements Properties of gamma-ray showers: Light distribution is fairly uniform Energy is evenly divided between bremsstrahlung photons and kinetic energy of e-s

56 Calibrating the Detector
PMT Gain Calibration Properties of gamma-ray showers: Light distribution is fairly uniform Energy is evenly divided between bremsstrahlung photons and kinetic energy of e-s electronics

57 STACEE AGN Targets Which objects are most scientifically promising?
R.A.Ong May 2005 Pulsar Nebula SNR AGN Other, UNID The Very High Energy Sky 3C 66A • z = 0.444 • Strong source at energies < 10 GeV • One questionable measurement at TeV energies • High redshift  heavy absorption • STACEE flux limit (Bramel et al. 2005) 3C 66A

58 STACEE AGN Targets Which objects are most scientifically promising?
Mkn 421 • z = 0.031 • Well-studied at TeV energies • Target of multi- variability studies • One previous measurement at GeV • High-energy peak is at ~100 GeV • Potential to constrain the optical EBL • STACEE detection (Carson 2005) R.A.Ong May 2005 Pulsar Nebula SNR AGN Other, UNID The Very High Energy Sky Mkn 421

59 STACEE AGN Targets Which objects are most scientifically promising?
W Comae • z = 0.102 • Hard EGRET spectrum:  = 1.73 • Limits only at TeV energies • STACEE observations can test model predictions • STACEE flux limit (Scalzo et al. 2004) R.A.Ong May 2005 Pulsar Nebula SNR AGN Other, UNID The Very High Energy Sky W Comae

60 Models of W Comae Predicted differences around 100 GeV Leptonic models
This source is interesting because it has a hard EGRET spectrum, but it hasn’t been detected at TeV energies. Also, there was detailed modeling of the source done, considering both leptonic and hadronic origins for the gamma-ray emission. Leptonic models no emission predicted above 100 GeV Hadronic models significant emission above 100 GeV

61 STACEE Measurement of W Comae
STACEE flux limit constrains hadronic emission models < ~ 2.5  cm-2 s-1 for hadronic models above 165 GeV Scalzo et al. 2004 More data taken last spring may lower these limits.

62 STACEE Measurement of W Comae
STACEE flux limit constrains hadronic emission models < ~ 2.5  cm-2 s-1 for hadronic models above 165 GeV Scalzo et al. 2004 More data taken last spring may lower these limits.

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